Why is the thread you use important and which to choose?
Basically you get what you pay for.
Check thread by running it between your pinched finger-tips it may feel slightly lumpy or look fluffy. If you buy a cheap thread its likely to break easily, the dye may come out and it could upset your sewing machine tension.
Thread on wooden bobbins looks wonderful but may prove too thick for your modern sewing machine. Always use the same thread in the shuttle as on the top (different colours are fine if that’s the look you want) the only exception is for some machine embroidery and top-stitching. Top-stitching is a decorative finish using a slightly thicker thread designed for this purpose. For most dressmaking fabrics a good quality polyester thread is perfect. If you’re using a pure cotton or silk fabric there are threads specifically for these.